This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The abbreviations and terms appearing in the description and drawings are defined as below.    3GPP Third Generation Partnership Project    BF Beamforming    BS Base Station    CSI Channel State Information    D2D Device-to-Device    DL Downlink    eNB enhanced NodeB, Base Station in E-UTRAN    FDD Frequency Division Duplex    IC Interference Cancellation    LTE Long Term Evolution    RB Resource Block    RS Reference Signal    TDD Time Division Duplex    UE User Equipment    UL Uplink
The device-to-device (D2D) communication strategy has attracted considerable attention as an underlay to current cellular networks. It enables user equipments (UE) to communicate with nearby user terminals directly over a D2D link without the help of cellular base stations (BS). The D2D strategy is particularly beneficial to serve the demand for high rate local data transmission and also capable of enhancing cell throughput and increasing spectral efficiency by reusing cellular resources.
Since the D2D link reuses the cellular frequency resource, the inter-channel interference (ICI) between the cellular and D2D links exists and sometimes the ICI can severely deteriorate the entire system performance. In order to mitigate the negative effect of ICI, a direct and intuitive way is to use orthogonal resource allocation, e.g., resource blocks (RB) assignment in LTE, for the cellular and the D2D links. It has been reported that the best performance by orthogonal resource allocation is achieved when transmit powers of both D2D transmitter and the cellular BS are maximized as they do not interfere with each other.
The orthogonal resource allocation schemes, however, do not fully achieve the frequency reuse gain, and hence lowers the spectrum efficiency. For performance enhancement, non-orthogonal frequency reuse strategies are recently investigated. For this case, a critical issue is to properly coordinate existing interference to guarantee both reliable macro cellular and D2D communications as they simultaneously reuse the same frequency and time resources.
To alleviate the cellular interference to the D2D links, interference cancellation (IC) precoding at the base station may be a good candidate. In “Interference-avoiding MIMO schemes for device-to-device radio underlaying cellular networks,” P. Janis, V. Koivunen, C. B. Ribeiro, K. Doppler, and K. Hugl, IEEE 20th International Symp. On Personal, Indoor and Mobile radio Commun. (PIMRC), December 2009, pp. 2385-2389, a novel interference cancellation (IC) precoder scheme for cellular DL transmission in the presence of D2D links is proposed. However, the IC scheme comes at the cost of reducing cellular users' signal power and hence decreases their achievable rates. In addition, the proposed IC scheme ignores the influences of path loss effects and limited feedback.
In contrast, eigen-beamforming (BF) scheme at the base station is capable of increasing cellular users' signal strength while causing more severe interference to D2D receivers at the same time.